Pressure sensor

ABSTRACT

A resistance welding apparatus comprises a piston having opposing faces for applying welding electrodes to a workpiece, means for measuring a pressure ratio across opposing faces of the piston and means for controlling the supply of welding current to the electrodes in dependence on the pressure ratio. A method of resistance welding and a pressure sensor for use in the resistance welding apparatus are also provided.

[0001] The present invention relates to a pressure sensor for use in, orin conjunction with resistance welding apparatus, to a method ofresistance welding and resistance welding apparatus.

[0002] Pneumatic cylinders (usually known as “guns”) are used in largenumbers as part of resistance welding apparatus in motor vehiclemanufacturing, and in the manufacture of white goods (washing machinesetc). Because of the high production volume it is important that thecyclic welding time is minimised. The preferred embodiments of thisinvention are directed to providing resistance welding apparatus capableof low cyclic times.

[0003] The present invention is particularly applicable to apparatuscomprising pneumatic cylinders. The word pneumatic is used herein tomean operable by a compressible fluid, usually but not necessarilycompressed air.

[0004] A pressure sensor according to the present invention canadvantageously be used in conjunction with the apparatus described inInternational Patent Application No. PCT/GB01/01700 filed on Apr. 12,2001 in the name of the present applicant, the entire disclosure ofwhich is herein incorporated by reference. A copy of this earlier PCTapplication is attached to this present application.

[0005] The preferred embodiments of the present invention are furtherdirected to accurate control of the timing of the application of thewelding current.

[0006] In one aspect the present invention provides a method ofresistance welding comprising monitoring a pressure ratio acrossopposing faces of a piston used to apply electrodes to a workpiece andcontrolling the supply of welding current to the electrodes independence on the pressure ratio.

[0007] Preferably, one said face of the piston is exposed to a pressuremoving the piston to apply the electrodes to the workpiece, and theother said face is exposed to a pressure of fluid being vented to permitmovement of the piston.

[0008] Preferably, the method comprises supplying the welding currentwhen the pressure ratio exceeds a predetermined value.

[0009] In a further aspect the present invention provides resistancewelding apparatus comprising a piston having opposing faces for applyingwelding electrodes to a workpiece, means for measuring a pressure ratioacross opposing faces of the piston and means for controlling the supplyof welding current to the electrodes in dependence on the pressureratio.

[0010] Preferably, the piston comprises a first said face to which, inoperation, pressure is applied to move the piston to apply theelectrodes to the workpiece, and another said face which, in operation,is exposed to a pressure in fluid being vented to permit movement of thepiston.

[0011] In a further aspect the present invention provides a pressuresensor for resistance welding apparatus, the sensor comprising a pistonhaving a first face and an oppositely-directed second face larger thanthe first face, means for applying to the first face pressure fluidutilised to effect closing movement of electrodes of the weldingapparatus to apply a welding force to a workpiece, means for applying tothe second face pressure fluid being vented to permit such electrodemovement, the relative areas of the first and second faces being chosensuch that the force exerted by pressure fluid applied to the first faceexceeds the force exerted by the pressure fluid acting on the secondface when the welding force applied to the workpiece is a predeterminedproportion of a maximum value, and means for controlling an electricalsignal in response to movement of the piston resulting from thedifference in forces applied to its first and second faces.

[0012] Preferably, the controlling means is a switch which is caused tochange state by movement of the piston, for example a proximity switch.

[0013] Preferred features of the present invention will now bedescribed, byway of example only, with reference to the accompanyingdrawings, in which:—

[0014]FIG. 1 shows a section through a pressure sensor according to thepresent invention in a first state;

[0015]FIG. 2 shows a section of the pressure sensor in a second state;

[0016]FIG. 3 shows a first embodiment of welding apparatus including apressure sensor according to the present invention in the first state;

[0017]FIG. 4 shows the apparatus of FIG. 3, the pressure sensor being inthe second state;

[0018]FIG. 5 shows a second embodiment of resistance welding apparatusincluding a pressure sensor according to the present invention in thefirst state;

[0019]FIG. 6 shows the apparatus of FIG. 5, the pressure sensor being inthe second state.

[0020] The pressure sensor 1 shown in FIGS. 1 and 2 comprises a piston 4moveable in pressure sensor body 2. As shown in FIGS. 1 and 2, piston 4is moveable towards the right and the left. It will be appreciated thatterms such as left, right, etc. are used for convenience having regardto the orientation of the device shown in the drawings. It may of coursebe operated in any convenient attitude.

[0021] Body 2 has a weld forward pilot port 14 and a weld return pilotport 15. Body 2 further defines a first pressure chamber 9 in fluidcommunication with weld forward pilot port 14 and a second pressurechamber 12 in fluid communication with weld return pilot port 15. Theright-hand piston face 8 of piston 4 sealingly bounds the secondpressure chamber 12, and the left-hand piston face 10 of piston 4sealingly bounds the first pressure chamber 9. The right-hand pistonface 8 is larger than the left-hand piston face 10. Movement of piston 4to the right is limited by flange 26 (when the right-hand piston face 8of piston 4 abuts against flange 26), and to the left by rear wall 24. Abore 11 of smaller cross section than the second pressure chamber 12extends from the rear wall 24 towards the left. The left-hand portion ofthe bore 11 forms the first pressure chamber 9. The left-hand portion ofpiston 4 is moveable within bore 11. A proximity switch 18 is located inor at the second pressure chamber 12. Its location is chosen such thatmovement of piston 4 from abutment at rear wall 24 towards its stop atflange 26 causes proximity switch 18 to change its state and that thereturn movement of piston 4 causes proximity switch 18 to change itsstate again. Conveniently the body 2 and piston 4 are of plasticsmaterial, the piston having a metal insert the approach of which issensed by the proximity switch 18. A suitable switch is sold by Pepperel& Fuchs.

[0022] A vent passage 16 extends from a space 25 bounded by rear wall 24the piston to atmosphere, to prevent air becoming compressed in thespace 25 and impeding movement of the piston.

[0023] Operation of the pressure sensor shown in FIGS. 1 and 2 will nowbe described. For the sake of simplicity some reference numerals shownin FIG. 1 have been omitted from FIG. 2.

[0024] Since the right-hand piston face 8 is larger (in the exampleshown substantially larger) than the left-hand piston face 10 the piston4 will, if the pressure supplied to weld return pilot port 15 is equalor larger than the pressure supplied to weld forward pilot port 14, bein its leftmost position. That is, the right-hand portion of piston 4will abut against rear wall 24. Piston 4 will remain in its leftmostposition provided the pressure at weld forward pilot port 14 is notgreater than the pressure at weld return pilot port 15 times the ratioof the area of the right-hand piston face 8 to the area of the left-handpiston face 10. When the pressure at weld forward pilot port 14 exceedsthis equilibrium pressure piston 4 will start to move towards the right.This moment is shown in FIG. 1. Piston 4 will continue to move towardsthe right until the right-hand piston face 8 abuts against flange 26 asshown in FIG. 2. As piston 4 moves towards the right air may pass fromthe atmosphere through vent passage 16 into the space 25 between theright-hand portion of piston 4 and rear wall 24.

[0025] Movement of the piston 4 towards the right causes proximityswitch 18 to change its state. This change of state can be used tocontrol the start of the application of welding current to the weldingelectrodes.

[0026] Subsequently, when the pressure at weld forward pilot port 14drops below the pressure at weld return pilot port 15 times the ratio ofthe area of the right-hand piston face 8 to the area of the left-handpiston face 10 piston 4 moves towards the left to its initial statewhere piston 4 abuts against rear wall 24. During this movement air fromthe space 25 passes through vent passage 16 to atmosphere. Further, thereturn movement of piston 4 causes proximity switch 18 to change itsstate again. The sensor 1 has then returned to its initial state readyfor the next welding cycle.

[0027] A first embodiment of resistance welding apparatus according tothe present invention is shown in FIGS. 3 and 4. The apparatus shown inFIG. 3 comprises a weld cylinder 28 of the three-port double-stroke typewhich has been described in sufficient detail in the aforementionedInternational Patent Application PCT/GB01/01700. Welding cylinder 28comprises first and second welding cylinder pistons 36 and 38. Thesecond welding cylinder piston 38 carries a welding electrode (notshown) which, upon rightward movement of the second welding cylinderpiston 38 can be applied against a workpiece so that the workpiece issqueezed between the welding electrode carried by piston 38 and a secondelectrode, for example a stationary electrode. During the first stroke(the forward stroke) of a welding cycle compressible fluid is suppliedfrom compressible fluid source 52 to forward port 30 of weldingcylinder28 via selector valve 54. This moves first and second weldingcylinder pistons 36 and 38 towards the right. After the forward strokecompressible fluid is supplied from source 52 to weld port 32 of weldingcylinder 28 via selector valve 56. This causes the second weldingcylinder piston 38 to move further towards the right (the first weldingcylinder piston 36 remaining stationary during this stroke). During thismovement compressible fluid is supplied into chamber 40 formed withinthe second welding cylinder piston 38. At the same time compressiblefluid is exhausted from chamber 42 bounded by the second weldingcylinder piston 38 and the housing of welding cylinder 28 via returnport 34. FIG. 4 shows the state of the welding apparatus at the end ofthe welding stroke, when the majority of compressible fluid has beenexhausted from chamber 42 via return port 34.

[0028] As shown in FIGS. 3 and 4, a pressure sensor such as the oneillustrated in FIGS. 1 and 2 is provided as part of the resistancewelding apparatus. Weld forward pilot port 14 of pressure sensor I isconnected by means of weld forward pilot line 20 to the line from port58 of selector valve 56 which is connected to weld port 32 of weldingcylinder 28. Weld return pilot port 15 is connected by means of weldreturn pilot line 22 to the line from port 60 of selector valve 56 toreturn port 34 of welding cylinder 28.

[0029] The operation of the resistance welding apparatus illustrated inFIGS. 3 and 4 will now be described. However, some details of theoperation which are conventional, or disclosed in the InternationalPatent Application referred to in the introduction of the presentapplication will not be described. During the first (the forward) strokecompressible fluid is supplied to forward port 30 by means of valve 54(FIG. 3). No compressible fluid is supplied to weld port 32, andcompressible fluid is exhausted from chamber 42 via return port 34. Theabsence of pressure at weld port 32 (and therefore also at weld forwardpilot port 14) and the presence of pressure at weld return pilot port 15due to the exhaustion of compressible fluid from chamber 42 causespiston 4 to remain at its leftmost position. During the weld stroke,before the electrode carried by the second welding cylinder piston 38abuts against the work piece (i.e. while the second welding cylinderpiston 38 moves towards the right with relatively little resistance) therelatively small pressure built-up at weld forward pilot port 14 is notsufficient to move piston 4 towards the right against the force exertedby the exhausting fluid from chamber 42 against the relatively largesurface of the right-hand piston face 8.

[0030] When the second welding cylinder piston 38 has reached theposition shown in FIG. 4 (the welding electrode carried by the secondwelding cylinder piston 38 abutting against the workpiece) pressurebuilds up in chamber 40 (and therefore also at weld forward pilot port14), while the pressure in chamber 42 decays (and therefore also at weldreturn pilot port 15). Once the ratio of the pressure at weld forwardpilot port 14 to that at weld return pilot port 15 is greater than theratio of the surface area of the right-hand piston face 8 to the surfacearea of the left-hand piston face 10 the piston 4 moves towards theright, causing proximity switch 18 to change its state as described inconnection with FIGS. 1 and 2. Hence the pressure sensor is used todetermine the build-up of the welding force applied to the work piece.Once this has reached a predetermined value, welding current is suppliedto the welding electrodes under the control of proximity switch 18.

[0031] The dimensions of the relevant parts of the resistance weldingapparatus (in particular the surface area ratio of the right-hand pistonface 8 to the left-hand piston face 10) may be chosen such that theproximity switch 18 changes its state once a desired percentage ofwelding force between the weld electrodes has been reached, for example80 or 90%. These dimensions can be determined, for example empirically.

[0032]FIGS. 5 and 6 show a second embodiment of resistance weldingapparatus according to the present invention including a pressure sensorsuch as the one described in connection with FIGS. 1 and 2. Large partsof the apparatus and its operation are identical to that described withreference to FIGS. 3 and 4 and will not be repeated here. The principaldifference between the apparatus shown in FIGS. 3 and 4 and that shownin FIGS. 5 and 6 is that the apparatus shown in FIGS. 5 and 6additionally comprises a return select valve 66 and a low impact valve110, both described in the International Patent ApplicationPCT/GB01/01700. As far as the operation of the pressure sensor 1 isconcerned, this embodiment operates in the same manner as that describedin connection with FIGS. 3 and 4.

[0033] It will be appreciated that the method of operation describedabove relies on the monitoring of a ratio of pressures in differentparts of resistance welding apparatus and controlling the supply ofwelding current to the electrodes in dependence on the pressure ratio.This can, of course, be achieved by means other than the pressure sensordescribed with reference to FIGS. 1 and 2. The various pressures could,for example, be measured separately (rather than their ratio) and themeasurement results processed, for example electronically, so as todetermine the moment at which supply of welding current to theelectrodes should be started.

[0034] It will also be appreciated that the pressure sensor can beprovided externally of the welding cylinder 28 (as illustrated in theattached drawings), or it could alternatively be incorporated into thepneumatic cylinder 28, preferably with internal connections between theweld port 32 to the weld forward pilot port 14 and the return port 34 tothe weld return pilot port 15. This results in a particularly compactdesign.

[0035] Whilst the present invention has been illustrated above withparticular reference to a three-port double-stroke type welding cylinderit will be appreciated that the invention may find application withtwo-port cylinders. In this case the forward port (also acting as weldport) would be connected to weld forward pilot port 14 of pressuresensor 1 and the return port 34 would be connected to the weld returnpilot port 15 of the pressure sensor 1. It is applicable also tofour-port (tandem) cylinders in which the chamber 40 is duplicated toincrease the electrode force.

[0036] While the present invention has been described in its preferredembodiments, it is to be understood that the words which have been usedare words of description rather than limitation and that changes may bemade to the invention without departing from its scope as defined by theappended claims.

[0037] Each feature disclosed in this specification (which term includesthe claims) and/or shown in the drawings may be incorporated in theinvention independently of other disclosed and/or illustrated features.

1. A method of resistance welding comprising monitoring a pressure ratio across opposing faces of a piston used to apply electrodes to a workpiece and controlling the supply of welding current to the electrodes in dependence on the pressure ratio.
 2. A method according to claim 1, wherein one said face of the piston is exposed to a pressure moving the piston to apply the electrodes to the workpiece, and the other said face is exposed to a pressure of fluid being vented to permit movement of the piston.
 3. A method according to claim 1 or claim 2, comprising supplying the welding current when the pressure ratio exceeds a predetermined value.
 4. Resistance welding apparatus comprising a piston having opposing faces for applying welding electrodes to a workpiece, means for measuring a pressure ratio across opposing faces of the piston and means for controlling the supply of welding current to the electrodes in dependence on the pressure ratio.
 5. Apparatus according to claim 4, wherein the piston comprises a first said face to which, in operation, pressure is applied to move the piston to apply the electrodes to the workpiece, and another said face which, in operation, is exposed to a pressure in fluid being vented to permit movement of the piston.
 6. A pressure sensor for resistance welding apparatus, the sensor comprising a piston having a first face and an oppositely-directed second face larger than the first face, means for applying to the first face pressure fluid utilised to effect closing movement of electrodes of the welding apparatus to apply a welding force to a workpiece, means for applying to the second face pressure fluid being vented to permit such electrode movement, the relative areas of the first and second faces being chosen such that the force exerted by pressure fluid applied to the first face exceeds the force exerted by the pressure fluid acting on the second face when the welding force applied to the workpiece is a predetermined proportion of a maximum value, and means for controlling an electrical signal in response to movement of the piston resulting from the difference in forces applied to its first and second faces.
 7. A sensor according to claim 6, wherein the controlling means is a switch which is caused to change state by movement of the piston.
 8. A sensor according to claim 7, wherein the switch is a proximity switch.
 9. A method, apparatus or sensor, substantially as herein described with reference to, or as illustrated in the accompanying drawings. 